Simple 3D printable car model which can be propelled by air balloon attached to flanged pipe on chassis part.

What differentiates this model from other similar is non-circular rear wheels that despite the shape allow this toy to ride smooth (you can read about Reuleaux triangle used to shape them in Wikipedia https://en.wikipedia.org/wiki/Reuleaux_triangle ). Front wheels support have some freedom for random direction change

Most likely axes will require some sandpaper applied to allow smooth rotation (also some pensil lead may be used as a graphite grease)